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Volcanological applications of SO2 cameras
Journal of Volcanology and Geothermal Research 300 (2015) 2–6
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Volcanological applications of SO2 cameras M.R. Burton a,⁎, F. Prata b, U. Platt c a b c
Istituto Nazionale di Geofisica e Vulcanologia, Pisa, Italy Climate & Atmosphere Department, NILU, Norway University of Heidelberg, Germany
a r t i c l e
i n f o
Article history: Received 5 June 2014 Accepted 19 September 2014 Available online 30 September 2014 Keywords: Plume imaging Volcanic plumes Remote sensing SO2 camera SO2 flux Volcano monitoring
a b s t r a c t Ground-based volcanic gas and ash imaging has the potential to revolutionise the way in which volcanoes are monitored and studied. The ability to track and quantify volcanic emissions in space and time with unprecedented fidelity opens the door to integration with geophysical measurements, allowing breakthroughs in our understanding of the physical processes driving volcanic activity. In May 2013 a European Science Foundation funded Plume Imaging workshop was conducted in Stromboli, Italy, with the objective of bringing the ground-based volcanic plume imaging community together in order to examine the state of the art, and move towards a ‘best-practice’ for volcanic ash and gas imaging techniques. A particular focus was the development of SO2 imaging systems, or SO2 cameras, with six teams deploying and testing various designs of ultraviolet and infrared-based imaging systems capable of imagining SO2. One conclusion of the workshop was that the term ‘SO2 camera’ should be applied to any SO2 imaging system, regardless of wavelength of radiation used. This Special Issue on Volcanic Plume Imaging is the direct result of the Stromboli workshop, and together the papers presented here represent the state of the art of ground-based volcano plume imaging science and technology. In this work, we examine in detail the volcanological applications of the SO2 camera, reviewing previous works and placing the new research contained in this Special Issue in context. The development of the SO2 camera, and future developments extending imaging to other volcanic gases, is one of the most exciting and novel research frontiers in volcanology today. © 2014 Published by Elsevier B.V.
1. Introduction In May 2013 a European Science Foundation-funded Plume Imaging workshop was conducted on Stromboli, Italy, with the objective of bringing the ground-based volcanic plume imaging community together in order to examine the state of the art, and move towards a ‘bestpractice’ for volcanic ash and gas imaging techniques of volcanic ash and gas imaging techniques. A particular focus was the development of SO2 imaging systems, or SO2 cameras, with six teams deploying and testing various designs of ultraviolet and infrared-based imaging systems capable of imaging SO2 (Kern et al., 2015a, b). This special issue is a direct product of that workshop, and represents the state of the art in volcanic gas and ash imaging research. The recent development of gas and ash imaging systems has opened up new frontiers in volcanology. The ability to detect rapid changes in gas and ash emissions allows a whole new series of phenomena to be examined over a range of scales, with a particular focus on explosive volcanism and the dynamics of passive degassing. Volcanologists may now combine comprehensive studies of gas emissions with seismic signals, producing unprecedented new constraints on the underlying ⁎ Corresponding author. E-mail address: [email protected] (M.R. Burton).
http://dx.doi.org/10.1016/j.jvolgeores.2014.09.008 0377-0273/© 2014 Published by Elsevier B.V.
physical processes that drive both geochemical and geophysical observations. Compared to earlier plume scanning technologies (e.g. Edmonds et al., 2003; Burton et al., 2007; Burton et al., 2009; Salerno et al., 2009; Galle et al., 2010) SO2 cameras allow more precise and much faster measurement of gas fluxes. Clearly, however, direct interdisciplinary comparisons also place great demands on the quality of data produced by each discipline. When used to make truly quantitative inferences on a physical process we require data that is both accurate, in order to e.g. compare with mass fluxes of lava, and precise, in order to distinguish real variations in flux from uncertainties. Furthermore, we begin a natural progression from short-term experimental field studies by research teams towards permanent and automatic data acquisition and analysis by volcano observatories. These demands require a rapid maturation of methodology, and the ongoing efforts of the community to ensure comparability of standards. In this paper we examine the role of the SO2 camera in volcanology and discuss what steps need to be made to fully realise the potential of the approach, highlighting how the papers in this special issue and previously published SO2 camera research have contributed to this goal. A detailed review of SO2 camera technologies is presented by Platt et al., 2015, and therefore in this work we focus on three main subjects: (i) Quantification of gas emission rates, (ii) volcanic processes which
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can be investigated with the SO2 camera, and (iii) implementation of SO2 cameras in monitoring networks. Platt et al. (2015) highlight that whilst the majority of SO2 cameras developed to date are based on ultraviolet spectroscopy, infrared spectroscopy offers tangible advantages, particularly in terms of night-time viewing. One conclusion of the workshop was the term ‘SO2 camera’ which should be applied to any SO2 imaging system, regardless of wavelength of radiation used. 2. Quantification of gas emission rates The majority of SO2 camera systems used to date (and 1-D plume scanning systems) are based on ultraviolet absorption spectroscopy using scattered sunlight as a source of radiation. This approach has been a staple of volcano observatories looking to monitor SO2 emissions from active volcanoes since the 1980s. As we discuss in more detail in Section 4, volcano observatory personnel are generally focussed on the interpretation of signals generated by their instruments, and tend to trust the results obtained by them. However, frequently it is overlooked that direct trace gas column measurements can be misleading unless appropriate corrections are applied (e.g. Lübcke et al., 2013). Light scattering below volcanic plumes can dilute the SO2 signal measured at the ground. This may be quite a significant problem, particularly when the distance between the instrument and the plume is large or the plume is ash-rich; unfortunately SO2 fluxes during eruptions are of great interest to volcano observatories. A theoretical treatment of these multiple scattering issues is described by Kern et al. (2013). Campion et al. (2015) use the brightness of the volcanic edifice to determine how much ‘light dilution’ is occurring, and use this to correct for the effect. Burton and Sawyer (2013) present an intensity-based spectrum analysis approach to solve the issue. Also, care has to be taken that the SO2 camera correctly calibrated, Lübcke et al. (2013) describe possible pitfalls in calibration and how problems can be avoided. More work is required to fully understand and resolve this challenging problem. A further major challenge in derivation of gas emission rates from SO2 camera data is the conversion of sequences of images of SO2 slant column amounts into a time series of SO2 flux. This was dealt within the first papers (e.g. Mori and Burton, 2006) by defining a line orthogonal to the plume advection direction and integrating the SO2 slant column amounts along that line, before multiplying by a plume velocity derived from the cross-correlation lag between parallel lines. This approach works well when the plume movement is quite uniform, which is a good approximation when the SO2 camera is far from the plume. However, one of the strengths of the SO2 camera is its spatial and temporal resolutions, which make it very often attractive to deploy it close to a source of gas, where the plume direction is far from uniform. In this case it is non-trivial to determine a flux, as there is no single velocity for the gas. Peters et al. (2015) address this problem using motion detection algorithms to find the overall flux of gas passing through a userdefined line or curve, allowing accurate determination of gas fluxes even close to the gas source. Kern et al. (2015a, b) report a very similar approach, used to help automate SO2 flux calculations on Kilauea. Validation of SO2 fluxes measured with the SO2 camera can also be obtained using a calibrated emission source, and such a test is reported by Smekens et al. (2015). A key element in the evolution of the SO2 camera is the intercomparison of the diverse approaches that have developed since the inception of the technique. The May 2013 plume imaging workshop on Stromboli provided the ideal vehicle for such an intercomparison, and the results from this are reported by Kern et al. (2015a, b). 3. Volcanic and degassing processes which can be investigated with the SO2 camera Magmatic degassing is one of the main drivers of volcanic processes, coupling with crystallisation and viscosity variations to determine the style and intensity of volcanic activity (Sparks, 2003). The magnitude
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of gas release is related to the mass of magma ascending in the volcanic system, allowing inferences on magma dynamics from mass balance considerations. In the following we review how SO2 cameras can improve our understanding of volcanic processes. 3.1. SO2 imagery and geophysical signals associated with volcanic activity One of the greatest strengths of the SO2 camera is its ability to measure fast changes in degassing rates at source, thanks to both, a potentially fast frame rate (cameras can collect up to 18 frames per second in ideal conditions) and the fact that each two-dimensional image records the history of degassing captured within the frame, like a ticker-tape timer. This ability makes the camera ideal for investigating the rates of degassing during explosions (see Section 3.2, below) and for studying the links between degassing and seismic and acoustic signals. Quantitative inversion of very long period (VLP) seismic events allows the volume change associated with the passage of a gas slug to be estimated (Chouet et al., 2003; Zuccarello et al., 2013). This information can be directly compared with results from the SO2 camera, in order to test our understanding of the physical processes which link gas slug ascent with VLP events. Volcanic tremor, an almost ubiquitous seismic signal on active volcanoes, is linked with magma and gas flow (e.g. Tamburello et al., 2013; Zuccarello et al., 2013), and therefore comparisons with gas flux data can deepen our understanding of the processes driving this seismic signal as well. We summarise the work performed to date comparing SO2 imagery with geophysical parameters below, and in Table 1. SO2 is typically a minor gas component in magmatic degassing, which is instead dominated by H2O and CO2. VLP signals are generated by magmatic gas, not just the SO2 component, and therefore the composition of the explosion gas must be known in order to convert from SO2 masses measured with the SO2 camera to total gas masses. In the future, technological developments may permit multiple gases to be imaged with a single instrument (see Platt et al., 2015). Dalton et al. (2010) were the first to study the relationship between acoustic signals and explosive gas release measured with an SO2 camera at Pacaya volcano, Guatemala. They found that gas masses derived from gas and acoustic datasets agreed to the same order of magnitude, and that the infrasound explosion records correlate with small pulses in degassing, but that longer-term degassing trends were probably controlled by a deeper process. The first SO2 camera comparisons with VLP signals were conducted by Kazahaya et al. (2011) and Nadeau et al. (2011), on Asama (Japan) and Fuego (Guatemala) volcanoes, respectively. Both authors found a linear correlation between SO2 amounts released in each explosion and VLP amplitude. Furthermore, Nadeau et al. (2011) found a clear correlation between volcanic tremor and gas emission rate, and suggested that decreases in gas emission rate may be the result of rheological stiffening in the upper conduit of Fuego. Tamburello et al. (2012) performed SO2 camera measurements on Stromboli, finding again a linear relationship between the VLP amplitude and explosion gas mass (2009) and extended their investigation to include thermal signals, which were also well-correlated with gas emissions. SO2 camera investigations of the degassing rate from Etna, Italy, show a correlation with volcanic tremor amplitude during passive degassing (Tamburello et al., 2013). Lopez et al. (2013) used an infrared imaging system, NicAIR, to measure SO2 and ash emission rates at Karymsky volcano, Kamchatka, which produced a wide range of degassing and explosive behaviours during their observation period. The NicAIR SO2 camera was used to quantify pulsatory magmatic degassing, but in this case no clear correlation with infrasound pressure or thermal variations were detected, probably due to the temporal delay between gas emission at the vent and SO2 detection downwind. In what is probably the most detailed study of the relationship between SO2 degassing rates measured with an SO2 camera and geophysical signals during explosive volcanic activity, Waite et al. (2013)
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Table 1 Reports of comparisons between SO2 camera-derived SO2 fluxes and geophysical parameters. Volcano Pacaya, Guatemala Asama, Japan Fuego, Guatemala Stromboli, Italy Etna, Italy Karymsky, Kamchatka Fuego, Guatemala Stromboli, Italy Stromboli, Italy Etna, Italy
Seismic VLP
Seismic tremor
Acoustic
Thermal
✓ ✓ ✓ ✓
✓ ✓ ✓
✓
✓ ✓ ✓ ✓
reported on further measurements collected on Fuego volcano, Guatemala. These data demonstrate that explosions are preceded by a clear decrease in SO2 flux, but not complete cessation, followed by a recovery in SO2 emission rates after the explosion. A VLP event was detected associated with each explosion. The authors compare the SO2 mass ratio/seismic moment (kg SO2 per N m) observed on Fuego (1.5–1.8 × 10−10 kg(SO2)/N m) with that reported by Kazahaya et al. (2011) for Asama (7.4 × 10−9 kg(SO2)/N m), and highlight that Fuego's ratio is ~50 times smaller. The difference could be due to a range of factors, including pressure changes and rock properties, but perhaps the largest uncertainty stems from a lack of knowledge of the mole fraction of SO2 in the explosion gas emissions. In this issue, three papers compare SO2 camera and geophysical data to investigate volcanic processes. Barnie et al. (2015) use SO2 camera and thermal camera data collected at the summit of Stromboli to quantify the relative masses of gas and solid emissions during explosions, finding from an initial survey that there is a poor overall correlation between gas and solid masses during explosions. Burton et al. (2015) report first results from a permanent SO2 flux monitoring SO2 camera, installed on Stromboli during the May 2013 Plume Imaging workshop. They investigate how the advected explosion gas arrives in the field of view of the SO2 camera at different speeds, dependent on wind direction, confirmed by simultaneous detection with a network of scanning UV spectrometers on the island, used to monitor SO2 emissions (Burton et al., 2009). Furthermore, they find a correlation between peak SO2 flux and integrated seismic energy release. Pering et al. (2015) use an SO2 camera to reveal gas emission rates during Strombolian activity at a vent in the summit area of Mt. Etna, Italy, but in this case no clear correlation with seismic events was detected. Ash emissions are often associated with explosive activity, and the presence of ash can strongly impact the ability of the SO2 camera to accurately quantify the SO2 abundance in explosions. In the works discussed in this section, authors have emphasised that they attempt to quantify ash-free explosions preferentially, in order to avoid ashcontamination issues. The reasons that these problems exist, and how to potentially solve them, are discussed in Section 2, Quantification of gas emission rates. In the case of discrete explosions, the SO2 camera has therefore demonstrated a great potential for bridging the gap between geophysics and gas geochemistry, allowing a direct comparison of explosive seismic events and explosion gas amounts, as well as quiescent degassing fluxes and volcanic tremor. One of the largest challenges lies in the conversion of SO2 masses to total gas masses, which must be made in order to achieve a physical comparison. Fortunately, techniques such as openpath FTIR can provide this information. It would be advantageous if future works adopted the use of the same seismic data product when comparing seismic events with SO2 fluxes, to facilitate comparison between datasets. This is complicated by the fact that, to first order, different frequencies in the seismic signals associated with gas-driven explosions reflect different processes, with low frequencies being coupled to volume changes and higher frequencies to overpressure. One solution may be to provide integrated absolute
Reference Dalton et al. (2010) Kazahaya et al. (2011) Nadeau et al. (2011) Tamburello et al. (2012) Tamburello et al. (2013) Lopez et al. (2013) Waite et al. (2013) Barnie et al. (2015) Burton et al. (2015) Peters et al. (2015)
amplitudes for all frequencies, low frequencies and high frequencies, respectively, for each seismic event. 3.2. Explosive versus passive degassing The combined capacity to measure gas emissions released over short timescales and from different sources allows the SO2 camera to distinguish and quantify the gas fluxes arising from different styles of volcanic activity. This opens the possibility of new insights into magma dynamics as the relative proportion of gas released during quiescent degassing compared with that in explosions is a direct reflection of the degree of coupling between gas and melt. A summary of the papers where the SO2 camera has been used to distinguish passive degassing fluxes from explosion gas fluxes is shown in Table 2. The first work with the SO2 camera to quantify the relative fluxes of SO2 released via explosive and quiescent degassing was performed by Mori and Burton (2009), who found that for Stromboli 3–8% of the total gas emission was released as explosions. There is an immediate conclusion from this observation: whilst explosions make a lot of noise and receive much attention, they are in fact a second order process compared with the quiescent supply of gas, and magma, to the near surface at Stromboli. This work was followed by that of Kazahaya et al. (2011) on Asama volcano Japan, who also found that the total gas flux was dominated by quiescent degassing, with 84% of SO2 emissions in between explosions. Holland et al. (2011) measured SO2 emissions from Santiaguito, and whilst they did not perform the calculation of the relative contribution of explosive and passive fluxes in that paper, the results they report allow such a calculation to be made. They measured background fluxes of 50 to 85 Mg/day (1 Mg is 1 metric tonne), and an average of ~25 explosions per day each releasing an average of ~650 kg, to produce a daily explosive flux of ~ 16 Mg/day or 19–32% of the total flux. Tamburello et al. (2012) furthered the investigation of degassing styles at Stromboli, quantifying both the emissions from quiescent degassing, ‘puffing’ and explosions. Gas ‘puffing’ behaviour is normally observed from the central of the three craters in the crater terrace of Stromboli, and is not normally associated with explosive activity, so it is a slightly pressurised and modulated quiescent degassing. They report relative proportions of 77% quiescent, 7% explosion and 16% ‘puffing’ SO2 fluxes, thus confirming the domination by quiescent degassing which makes up 93% of the total flux, as reported by Mori and Burton (2009). Waite et al. (2013) report data which allow the relative calculation of quiescent to explosive degassing, and find that on this volcano, anomalously compared with other studies, degassing is dominated by gas released during explosions. From inspection of Figure 4 in Waite et al. (2013) the average total flux over 70 min was of ~1.25 kg/s, producing a total of 5250 kg SO2, whilst 6 explosions in the same period produced 5028 kg, indicating that ~95% of the degassing comes from explosions. Pering et al. (2015) report on SO2 fluxes from mild explosive activity from a vent in the Central Crater of Etna, Italy, whose values are orders of magnitude smaller than the quiescent degassing flux from the main craters of this volcano.
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Table 2 Reports of comparisons between explosive and quiescent degassing fluxes with the SO2 camera. Volcano
Quiescent gas flux %
Explosive gas flux %
Reference
Stromboli, Italy Asama, Japan Santiaguito, Guatemala Stromboli, Italy Fuego, Guatemala Etna, Italy
92–97% 84% 68–81% 93% ~5% N99%
3–8% 16% 19–32% 7% ~95% b1%
Mori and Burton (2009) Kazahaya et al. (2011) Holland et al. (2011) Tamburello et al. (2012) Waite et al. (2013) Peters et al. (2015)
3.3. Comparison of inferred mass of degassed magma with mass of erupted magma One of the earliest uses of SO2 flux measurements, after simple pattern recognition to indicate magmatic unrest, was the quantification of the mass of magma required to produce an observed gas flux. This inferred degassed magma mass can then be compared with the mass of erupted magma. A smaller mass of degassed magma than erupted magma may imply incomplete degassing of magma, but this is very rarely observed. Much more common is the opposite case, where more magma is degassed than is erupted, implying permanent storage of degassed magma under the volcano, leading to endogenous growth. The first work to utilise these mass balance considerations was that of Francis et al. (1993) who examined endogenous growth for Etna, Italy and Kilauea, Hawaii. A key requirement for such calculations is knowledge of the original concentration of sulphur in the melt prior to degassing. This information can best be obtained by analysis of melt inclusions trapped in crystals that grew at high pressure before being rapidly transported to the surface in an eruption. We highlight therefore that the scientific insights which can be gained from SO2 flux measurements can be further enhanced when integrated with other measurements, in this case of the volatile contents of melt inclusions in erupted crystals. Whilst such data exist for some well-studied volcanoes, there are many degassing volcanoes that have not yet been examined for primitive volatile amounts, and therefore such studies should be a priority in order to fully utilise information obtained from the SO2 camera.
3.4. Lava lake dynamics There are four lava lakes recognised on Earth, Erta'Ale in Ethiopa, Nyiragongo in Democratic Republic of Congo, Halemaumau on Kilauea, Hawaii, and Erebus in Antarctica. Lava lakes provide fascinating insights into the dynamics of persistently active volcanoes, revealing styles of degassing, magma supply and magma level which are normally hidden from view. In this issue two papers report the first SO2 camera measurements of SO2 flux from a lava lake, focussing on the Halemaumau crater of Kilauea. Nadeau et al. (2015) examine two examples of lava lake rise and fall. When the lava levels were high, diminished SO2 emissions and volcanic tremor were observed; in contrast, when lava levels were low, degassing rates were higher. The high lava levels were interpreted as resulting from the accumulation of gas below a low-permeability lava lake surface. The recent installation of a permanent SO2 camera to monitor the lava lake at Kilaeua (Kern et al., 2015a, b, see Section 4) will provide an unprecedented insight into the dynamics of this quite rare phenomenon.
Stremme et al. (2012) used the scanning infrared emission spectrometer based SIGIS system from Bruker optics (www.bruker.de) to measure the SiF4/SO2 ratio in the volcanic plume of Popocatepetl in Mexico. They found distinct trends related to volcanic activity, with SiF4 increasing relative to SO2 prior to an explosion on the volcano, and then decreasing thereafter. The passive, thermal emission spectroscopy upon which the SIGIS system is based can be used during the day and the night, a major advantage over UV-based systems which require scattered sunlight. Platt et al. (2015) give a full analysis of the various technologies that can be used in SO2 camera systems. BrO/SO2 and OClO/SO2 ratios are reported by General et al. (2015), who used the IDOAS approach to detect these difficult-to-measure volcanic gases. Pering et al. (2014) simultaneously measured gas compositions and SO2 fluxes with MultiGas and SO2 camera instruments respectively on Mt. Etna (Italy). Lopez et al. (2013) report ash quantification in an explosion from Karymsky (Kamchatka), using the infrared imaging system NicAIR. This work was extended by Lopez et al. (2015), examining ash and SO2 quantifications on both Stromboli (Italy) and Karymsky (Russia). Cerminara et al. (2015) examine the integration of thermal imagery with a numerical model to determine ash loading in explosions at Stromboli. 4. Implementation of SO2 cameras in monitoring networks As described in Section 3, whilst flux determinations may appear simple in theory, in practice there are many challenges in producing the accurate and precise SO2 slant column amounts and fluxes required to perform useful comparisons with geophysical data (Section 3.1) and erupted magma masses (Section 3.3). However, volcano observatories require instruments that can be relied upon to produce good-quality data, without the need for experts in radiative transfer. There is therefore a tension between what is actually required by volcano observatories, and what is currently provided by SO2 flux monitoring systems that currently do not resolve a host of issues related to plume geometry, plume speed and multiple scattering. Furthermore, in an ideal world, volcano observatories would have 24-hour SO2 flux monitoring capabilities, which cannot be provided by the UV-based SO2 cameras (or scanning systems, see Galle et al., 2010), which make up the majority of instruments to date. An investment in infrared-based technique development (or other techniques, see Platt et al., 2015) may well yield great benefits for volcano observatories, greatly extending their monitoring capacities. The first fully automated SO2 camera systems have been deployed on two volcanoes to date, Kilauea (Kern et al., 2015a, b) and Stromboli (Burton et al., 2015). These imaging systems represent a stepping stone to the future of SO2 flux monitoring at volcano observatories, and are already producing unprecedentedly detailed insights into the degassing processes at both volcanoes.
3.5. Gas composition and ash detection/quantification 5. Conclusions As well as revealing the SO2 flux from volcanoes, infrared-based SO2 cameras can also be used to determine plume gas compositions and ash burdens, whilst scanning 2D UV spectrometers can be used to measure several trace volcanic gases (like BrO and OClO).
The development of the SO2 camera is a major step forward for volcano research, enabling comparison of quantified gas emissions with geophysical data (see Section 3.1) and erupted masses (Section 3.3).
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Such comparisons require precise and accurate measurements of SO2 flux, which are far from trivial to achieve, due to the challenges of light scattering and complex flow within the advecting plume. Solutions to these problems are beginning to be within our reach however (see Section 2). Multi-spectral imaging can open the door to even more detailed studies including the quantification of gases other than SO2, allowing examination of plume gas chemistry (see Section 3.5). Some of the major benefits of SO2 flux monitoring with SO2 cameras arise from comparisons between the flux of SO2 released explosively versus that released passively, as this reflects on the coupling between gas and magma, a major controlling factor in the overall behaviour of a volcano (Section 3.2). The SO2 camera has revolutionised our ability to monitor volcanic gas fluxes, and a thriving community has arisen around this technology. The May 2013 ESF-funded MeMoVolc workshop has allowed this community to come together in the same place for the first time, and resulted in this special issue, which reflects the excitement and potential provided by this new technique. Acknowledgements We gratefully acknowledge the European Science Foundation which provided the funding to the MeMoVolc project, kindly managed by Prof. Tim Druitt and Dr. Augusto Neri, whom we thank for their support in creating the Plume Imaging Workshop, grant number 4799. We also thank all the participants of the Plume Imaging Workshop for making it such a stimulating and productive event, and for their continued enthusiasm since then, which has resulted in this Special Issue. Prof. Lionel Wilson and Prof. Clive Oppenheimer are thanked for their respective editing and revision of this work. We thank Patrizia Pantani, Raffaella Pignolo and Gemma Prata for their assistance in organising the workshop. The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement no. [279802]. References Barnie, T., Bombrun, M., Burton, M.R., Harris, A.J.L., Sawyer, G., 2015. Quantification of gas and solid emissions during Strombolian explosions using simultaneous sulphur dioxide and infrared camera observations. J. Volcanol. Geotherm. Res. 300, 167–174. Burton, M.R., Sawyer, G., 2013. iFit and light dilution: ultraviolet volcanic SO2 measurements under the microscope. EGU General Assembly Conference Abstracts, p. 10715. Burton, M., Allard, P., Mure, F., La Spina, A., 2007. Magmatic gas composition reveals the source depth of slug-driven Strombolian explosive activity. Science 317 (5835), 227–230. Burton, M.R., Caltabiano, T., Mure, F., Salerno, G., Randazzo, D., 2009. SO2 flux from Stromboli during the 2007 eruption: results from the FLAME network and traverse measurements. J. Volcanol. Geotherm. Res. 182 (3–4), 214–220. Burton, M.R., Salerno, G.G., D'Auria, L., 2015. SO2 flux monitoring at Stromboli with the new permanent INGV SO2 camera system: A comparison with the FLAME network and seismological data. J. Volcanol. Geotherm. Res. 300, 95–102. Campion, R.A., Delgado-Granados, H., Mori, T., 2015. Image-based Correction of the Light Dilution Effect for SO2 Camera Measurements. J. Volcanol. Geotherm. Res. 300, 48–57. Cerminara, M., Ongaro, T.E., Valade, S.A., Harris, A.J.L., 2015. Volcanic plume vent conditions retrieved from infrared images: A forward and inverse modeling approach. J. Volcanol. Geotherm. Res. 300, 129–147. Chouet, B., Dawson, P., Ohminato, T., Martini, M., Saccorotti, G., Giudicepietro, F., De Luca, G., Milana, G., Scarpa, R., 2003. Source mechanism of explosions at Stromboli Volcano, Italy, determined from moment-tensor inversions of very-long-period data. J. Geophys. Res. 108 (B1), 2019. http://dx.doi.org/10.1029/20042JB001919. Dalton, M.P., Waite, G.P., Watson, I.M., Nadeau, P.A., 2010. Multiparameter quantification of gas release during weak Strombolian eruptions at Pacaya Volcano, Guatemala. Geophys. Res. Lett. 37. Edmonds, M., Herd, R.A., Galle, B., Oppenheimer, C.M., 2003. Automated, high time resolution measurements of SO2 flux at Soufriere Hills Volcano, Montserrat. Bull. Volcanol. 65, 578–586.
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Relation between single very-long-period pulses and volcanic gas emissions at Mt. Asama, Japan. Geophys. Res. Lett. 38. Kern, C., Werner, C., Elias, T., Sutton, A.J., Lubcke, P., 2013. Applying UV cameras for SO2 detection to distant or optically thick volcanic plumes. J. Volcanol. Geotherm. Res. 262, 80–89. Kern, C., Sutton, A.J., Elias, T., Lee, L., Kamibayashi, K., Antolik, L., Wener, C., 2015a. An SO2 Camera System for Continuous, Real-time Monitoring of Gas Emissions From Kīlauea Volcano's Summit Overlook Crater. J. Volcanol. Geotherm. Res. 300, 81–94. Kern, C., et al., 2015b. Intercomparison of SO2 Camera Systems for Imaging Volcanic Gas Plumes. J. Volcanol. Geotherm. Res. 300, 22–36. Lopez, T., Fee, D., Prata, F., Dehn, J., 2013. Characterization and interpretation of volcanic activity at Karymsky Volcano, Kamchatka, Russia, using observations of infrasound, volcanic emissions, and thermal imagery. Geochem. Geophys. Geosyst. 14 (12), 5106–5127. Lopez, T., Thomas, H., Prata, F., Amigo, A., Fee, D., Moriano, D., 2015. Volcanic Plume Characteristics Determined Using an Infrared Imaging Camera. J. Volcanol. Geotherm. Res. 300, 148–166. Lübcke, P., Bobrowski, N., Illing, S., Kern, C., Alvarez Nieves, J.M., Vogel, L., Zielcke, J., Delgado, Granados H., Platt, U., 2013. On the absolute calibration of SO2 cameras. Atmos. Meas. Tech. 6 (3), 677–696. http://dx.doi.org/10.5194/amt-6-677-2013. Mori, T., Burton, M., 2006. The SO2 camera: a simple, fast and cheap method for groundbased imaging of SO2 in volcanic plumes. Geophys. Res. Lett. 33 (24). Mori, T., Burton, M., 2009. Quantification of the gas mass emitted during single explosions on Stromboli with the SO2 imaging camera. J. Volcanol. Geotherm. Res. 188 (4), 395–400. Nadeau, P.A., Palma, J.L., Waite, G.P., 2011. Linking volcanic tremor, degassing, and eruption dynamics via SO2 imaging. Geophys. Res. Lett. 38. Nadeau, P.A., Werner, C., Waite, G.P., Carn, S.A., Brewer, I.D., Elias, T., Suttom, A.J., Kern, C., 2015. Using SO2 Camera Imagery to Examine Degassing and Gas Accumulation at Kilauea Volcano. J. Volcanol. Geotherm. Res. 300, 70–80. Pering, T.D., et al., 2015. Dynamics of Mild Strombolian Activity on Mt. Etna. J. Volcanol. Geotherm. Res. 300, 103–111. Pering, T.D., Tamburello, G., McGonigle, A.J.S., Aiuppa, A., Cannata, A., Giudice, G., Patane, D., 2014. High time resolution fluctuations in volcanic carbon dioxide degassing from Mount Etna. J. Volcanol. Geotherm. Res. 270, 115–121. Peters, N., Hoffmann, A., Barnie, T., Herzog, M., Oppenheimer, C., 2015. Use of Motion Estimation Algorithms for Improved SO2 Measurements Using UV Cameras. J. Volcanol. Geotherm. Res. 300, 58–69. Platt, U., Lübcke, P., Kuhn, J., Bobrowski, N., Prata, F., Burton, M.R., Kern, C., 2015. Quantitative Imaging of Volcanic Plumes — Results, Future Needs, and Future Trends. J. Volcanol. Geotherm. Res. 300, 7–21. Salerno, G.G., Burton, M.R., Oppenheimer, C., Caltabiano, T., Randazzo, D., Bruno, N., Longo, V., 2009. Three-years of SO2 flux measurements of Mt. Etna using an automated UV scanner array: comparison with conventional traverses and uncertainties in flux retrieval. J. Volcanol. Geotherm. Res. 183 (1–2), 76–83. Smekens, J.F., Burton, M.R., Clarke, A.B., 2015. Validation of the SO2 Camera for High Temporal and Spatial Resolution Monitoring of SO2 Emissions. J. Volcanol. Geotherm. Res. 300, 37–47. Sparks, R.S.J., 2003. Forecasting volcanic eruptions. Earth Planet. Sci. Lett. 210, 1–15. Stremme, W., Krueger, A., Harig, R., Grutter, M., 2012. Volcanic SO2 and SiF4 visualization using 2-D thermal emission spectroscopy — part 1: slant-columns and their ratios. Atmos. Meas. Tech. 5, 275–288. Tamburello, G., Aiuppa, A., Kantzas, E.P., McGonigle, A.J.S., Ripepe, M., 2012. Passive vs. active degassing modes at an open-vent volcano (Stromboli, Italy). Earth Planet. Sci. Lett. 359, 106–116. Tamburello, G., Aiuppa, A., McGonigle, A.J.S., Allard, P., Cannata, A., Giudice, G., Kantzas, E.P., Pering, T.D., 2013. Periodic volcanic degassing behavior: the Mount Etna example. Geophys. Res. Lett. 40 (18), 4818–4822. Waite, G.P., Nadeau, P.A., Lyons, J.J., 2013. Variability in eruption style and associated very long period events at Fuego volcano, Guatemala. J. Geophys. Res. Solid Earth 118 (4), 1526–1533. Zuccarello, L., Burton, M.R., Saccorotti, G., Bean, C.J., Patane, D., 2013. The coupling between very long period seismic events, volcanic tremor, and degassing rates at Mount Etna volcano. J. Geophys. Res. Solid Earth 118 (9), 4910–4921.
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Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores
Volcanological applications of SO2 cameras M.R. Burton a,⁎, F. Prata b, U. Platt c a b c
Istituto Nazionale di Geofisica e Vulcanologia, Pisa, Italy Climate & Atmosphere Department, NILU, Norway University of Heidelberg, Germany
a r t i c l e
i n f o
Article history: Received 5 June 2014 Accepted 19 September 2014 Available online 30 September 2014 Keywords: Plume imaging Volcanic plumes Remote sensing SO2 camera SO2 flux Volcano monitoring
a b s t r a c t Ground-based volcanic gas and ash imaging has the potential to revolutionise the way in which volcanoes are monitored and studied. The ability to track and quantify volcanic emissions in space and time with unprecedented fidelity opens the door to integration with geophysical measurements, allowing breakthroughs in our understanding of the physical processes driving volcanic activity. In May 2013 a European Science Foundation funded Plume Imaging workshop was conducted in Stromboli, Italy, with the objective of bringing the ground-based volcanic plume imaging community together in order to examine the state of the art, and move towards a ‘best-practice’ for volcanic ash and gas imaging techniques. A particular focus was the development of SO2 imaging systems, or SO2 cameras, with six teams deploying and testing various designs of ultraviolet and infrared-based imaging systems capable of imagining SO2. One conclusion of the workshop was that the term ‘SO2 camera’ should be applied to any SO2 imaging system, regardless of wavelength of radiation used. This Special Issue on Volcanic Plume Imaging is the direct result of the Stromboli workshop, and together the papers presented here represent the state of the art of ground-based volcano plume imaging science and technology. In this work, we examine in detail the volcanological applications of the SO2 camera, reviewing previous works and placing the new research contained in this Special Issue in context. The development of the SO2 camera, and future developments extending imaging to other volcanic gases, is one of the most exciting and novel research frontiers in volcanology today. © 2014 Published by Elsevier B.V.
1. Introduction In May 2013 a European Science Foundation-funded Plume Imaging workshop was conducted on Stromboli, Italy, with the objective of bringing the ground-based volcanic plume imaging community together in order to examine the state of the art, and move towards a ‘bestpractice’ for volcanic ash and gas imaging techniques of volcanic ash and gas imaging techniques. A particular focus was the development of SO2 imaging systems, or SO2 cameras, with six teams deploying and testing various designs of ultraviolet and infrared-based imaging systems capable of imaging SO2 (Kern et al., 2015a, b). This special issue is a direct product of that workshop, and represents the state of the art in volcanic gas and ash imaging research. The recent development of gas and ash imaging systems has opened up new frontiers in volcanology. The ability to detect rapid changes in gas and ash emissions allows a whole new series of phenomena to be examined over a range of scales, with a particular focus on explosive volcanism and the dynamics of passive degassing. Volcanologists may now combine comprehensive studies of gas emissions with seismic signals, producing unprecedented new constraints on the underlying ⁎ Corresponding author. E-mail address: [email protected] (M.R. Burton).
http://dx.doi.org/10.1016/j.jvolgeores.2014.09.008 0377-0273/© 2014 Published by Elsevier B.V.
physical processes that drive both geochemical and geophysical observations. Compared to earlier plume scanning technologies (e.g. Edmonds et al., 2003; Burton et al., 2007; Burton et al., 2009; Salerno et al., 2009; Galle et al., 2010) SO2 cameras allow more precise and much faster measurement of gas fluxes. Clearly, however, direct interdisciplinary comparisons also place great demands on the quality of data produced by each discipline. When used to make truly quantitative inferences on a physical process we require data that is both accurate, in order to e.g. compare with mass fluxes of lava, and precise, in order to distinguish real variations in flux from uncertainties. Furthermore, we begin a natural progression from short-term experimental field studies by research teams towards permanent and automatic data acquisition and analysis by volcano observatories. These demands require a rapid maturation of methodology, and the ongoing efforts of the community to ensure comparability of standards. In this paper we examine the role of the SO2 camera in volcanology and discuss what steps need to be made to fully realise the potential of the approach, highlighting how the papers in this special issue and previously published SO2 camera research have contributed to this goal. A detailed review of SO2 camera technologies is presented by Platt et al., 2015, and therefore in this work we focus on three main subjects: (i) Quantification of gas emission rates, (ii) volcanic processes which
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can be investigated with the SO2 camera, and (iii) implementation of SO2 cameras in monitoring networks. Platt et al. (2015) highlight that whilst the majority of SO2 cameras developed to date are based on ultraviolet spectroscopy, infrared spectroscopy offers tangible advantages, particularly in terms of night-time viewing. One conclusion of the workshop was the term ‘SO2 camera’ which should be applied to any SO2 imaging system, regardless of wavelength of radiation used. 2. Quantification of gas emission rates The majority of SO2 camera systems used to date (and 1-D plume scanning systems) are based on ultraviolet absorption spectroscopy using scattered sunlight as a source of radiation. This approach has been a staple of volcano observatories looking to monitor SO2 emissions from active volcanoes since the 1980s. As we discuss in more detail in Section 4, volcano observatory personnel are generally focussed on the interpretation of signals generated by their instruments, and tend to trust the results obtained by them. However, frequently it is overlooked that direct trace gas column measurements can be misleading unless appropriate corrections are applied (e.g. Lübcke et al., 2013). Light scattering below volcanic plumes can dilute the SO2 signal measured at the ground. This may be quite a significant problem, particularly when the distance between the instrument and the plume is large or the plume is ash-rich; unfortunately SO2 fluxes during eruptions are of great interest to volcano observatories. A theoretical treatment of these multiple scattering issues is described by Kern et al. (2013). Campion et al. (2015) use the brightness of the volcanic edifice to determine how much ‘light dilution’ is occurring, and use this to correct for the effect. Burton and Sawyer (2013) present an intensity-based spectrum analysis approach to solve the issue. Also, care has to be taken that the SO2 camera correctly calibrated, Lübcke et al. (2013) describe possible pitfalls in calibration and how problems can be avoided. More work is required to fully understand and resolve this challenging problem. A further major challenge in derivation of gas emission rates from SO2 camera data is the conversion of sequences of images of SO2 slant column amounts into a time series of SO2 flux. This was dealt within the first papers (e.g. Mori and Burton, 2006) by defining a line orthogonal to the plume advection direction and integrating the SO2 slant column amounts along that line, before multiplying by a plume velocity derived from the cross-correlation lag between parallel lines. This approach works well when the plume movement is quite uniform, which is a good approximation when the SO2 camera is far from the plume. However, one of the strengths of the SO2 camera is its spatial and temporal resolutions, which make it very often attractive to deploy it close to a source of gas, where the plume direction is far from uniform. In this case it is non-trivial to determine a flux, as there is no single velocity for the gas. Peters et al. (2015) address this problem using motion detection algorithms to find the overall flux of gas passing through a userdefined line or curve, allowing accurate determination of gas fluxes even close to the gas source. Kern et al. (2015a, b) report a very similar approach, used to help automate SO2 flux calculations on Kilauea. Validation of SO2 fluxes measured with the SO2 camera can also be obtained using a calibrated emission source, and such a test is reported by Smekens et al. (2015). A key element in the evolution of the SO2 camera is the intercomparison of the diverse approaches that have developed since the inception of the technique. The May 2013 plume imaging workshop on Stromboli provided the ideal vehicle for such an intercomparison, and the results from this are reported by Kern et al. (2015a, b). 3. Volcanic and degassing processes which can be investigated with the SO2 camera Magmatic degassing is one of the main drivers of volcanic processes, coupling with crystallisation and viscosity variations to determine the style and intensity of volcanic activity (Sparks, 2003). The magnitude
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of gas release is related to the mass of magma ascending in the volcanic system, allowing inferences on magma dynamics from mass balance considerations. In the following we review how SO2 cameras can improve our understanding of volcanic processes. 3.1. SO2 imagery and geophysical signals associated with volcanic activity One of the greatest strengths of the SO2 camera is its ability to measure fast changes in degassing rates at source, thanks to both, a potentially fast frame rate (cameras can collect up to 18 frames per second in ideal conditions) and the fact that each two-dimensional image records the history of degassing captured within the frame, like a ticker-tape timer. This ability makes the camera ideal for investigating the rates of degassing during explosions (see Section 3.2, below) and for studying the links between degassing and seismic and acoustic signals. Quantitative inversion of very long period (VLP) seismic events allows the volume change associated with the passage of a gas slug to be estimated (Chouet et al., 2003; Zuccarello et al., 2013). This information can be directly compared with results from the SO2 camera, in order to test our understanding of the physical processes which link gas slug ascent with VLP events. Volcanic tremor, an almost ubiquitous seismic signal on active volcanoes, is linked with magma and gas flow (e.g. Tamburello et al., 2013; Zuccarello et al., 2013), and therefore comparisons with gas flux data can deepen our understanding of the processes driving this seismic signal as well. We summarise the work performed to date comparing SO2 imagery with geophysical parameters below, and in Table 1. SO2 is typically a minor gas component in magmatic degassing, which is instead dominated by H2O and CO2. VLP signals are generated by magmatic gas, not just the SO2 component, and therefore the composition of the explosion gas must be known in order to convert from SO2 masses measured with the SO2 camera to total gas masses. In the future, technological developments may permit multiple gases to be imaged with a single instrument (see Platt et al., 2015). Dalton et al. (2010) were the first to study the relationship between acoustic signals and explosive gas release measured with an SO2 camera at Pacaya volcano, Guatemala. They found that gas masses derived from gas and acoustic datasets agreed to the same order of magnitude, and that the infrasound explosion records correlate with small pulses in degassing, but that longer-term degassing trends were probably controlled by a deeper process. The first SO2 camera comparisons with VLP signals were conducted by Kazahaya et al. (2011) and Nadeau et al. (2011), on Asama (Japan) and Fuego (Guatemala) volcanoes, respectively. Both authors found a linear correlation between SO2 amounts released in each explosion and VLP amplitude. Furthermore, Nadeau et al. (2011) found a clear correlation between volcanic tremor and gas emission rate, and suggested that decreases in gas emission rate may be the result of rheological stiffening in the upper conduit of Fuego. Tamburello et al. (2012) performed SO2 camera measurements on Stromboli, finding again a linear relationship between the VLP amplitude and explosion gas mass (2009) and extended their investigation to include thermal signals, which were also well-correlated with gas emissions. SO2 camera investigations of the degassing rate from Etna, Italy, show a correlation with volcanic tremor amplitude during passive degassing (Tamburello et al., 2013). Lopez et al. (2013) used an infrared imaging system, NicAIR, to measure SO2 and ash emission rates at Karymsky volcano, Kamchatka, which produced a wide range of degassing and explosive behaviours during their observation period. The NicAIR SO2 camera was used to quantify pulsatory magmatic degassing, but in this case no clear correlation with infrasound pressure or thermal variations were detected, probably due to the temporal delay between gas emission at the vent and SO2 detection downwind. In what is probably the most detailed study of the relationship between SO2 degassing rates measured with an SO2 camera and geophysical signals during explosive volcanic activity, Waite et al. (2013)
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Table 1 Reports of comparisons between SO2 camera-derived SO2 fluxes and geophysical parameters. Volcano Pacaya, Guatemala Asama, Japan Fuego, Guatemala Stromboli, Italy Etna, Italy Karymsky, Kamchatka Fuego, Guatemala Stromboli, Italy Stromboli, Italy Etna, Italy
Seismic VLP
Seismic tremor
Acoustic
Thermal
✓ ✓ ✓ ✓
✓ ✓ ✓
✓
✓ ✓ ✓ ✓
reported on further measurements collected on Fuego volcano, Guatemala. These data demonstrate that explosions are preceded by a clear decrease in SO2 flux, but not complete cessation, followed by a recovery in SO2 emission rates after the explosion. A VLP event was detected associated with each explosion. The authors compare the SO2 mass ratio/seismic moment (kg SO2 per N m) observed on Fuego (1.5–1.8 × 10−10 kg(SO2)/N m) with that reported by Kazahaya et al. (2011) for Asama (7.4 × 10−9 kg(SO2)/N m), and highlight that Fuego's ratio is ~50 times smaller. The difference could be due to a range of factors, including pressure changes and rock properties, but perhaps the largest uncertainty stems from a lack of knowledge of the mole fraction of SO2 in the explosion gas emissions. In this issue, three papers compare SO2 camera and geophysical data to investigate volcanic processes. Barnie et al. (2015) use SO2 camera and thermal camera data collected at the summit of Stromboli to quantify the relative masses of gas and solid emissions during explosions, finding from an initial survey that there is a poor overall correlation between gas and solid masses during explosions. Burton et al. (2015) report first results from a permanent SO2 flux monitoring SO2 camera, installed on Stromboli during the May 2013 Plume Imaging workshop. They investigate how the advected explosion gas arrives in the field of view of the SO2 camera at different speeds, dependent on wind direction, confirmed by simultaneous detection with a network of scanning UV spectrometers on the island, used to monitor SO2 emissions (Burton et al., 2009). Furthermore, they find a correlation between peak SO2 flux and integrated seismic energy release. Pering et al. (2015) use an SO2 camera to reveal gas emission rates during Strombolian activity at a vent in the summit area of Mt. Etna, Italy, but in this case no clear correlation with seismic events was detected. Ash emissions are often associated with explosive activity, and the presence of ash can strongly impact the ability of the SO2 camera to accurately quantify the SO2 abundance in explosions. In the works discussed in this section, authors have emphasised that they attempt to quantify ash-free explosions preferentially, in order to avoid ashcontamination issues. The reasons that these problems exist, and how to potentially solve them, are discussed in Section 2, Quantification of gas emission rates. In the case of discrete explosions, the SO2 camera has therefore demonstrated a great potential for bridging the gap between geophysics and gas geochemistry, allowing a direct comparison of explosive seismic events and explosion gas amounts, as well as quiescent degassing fluxes and volcanic tremor. One of the largest challenges lies in the conversion of SO2 masses to total gas masses, which must be made in order to achieve a physical comparison. Fortunately, techniques such as openpath FTIR can provide this information. It would be advantageous if future works adopted the use of the same seismic data product when comparing seismic events with SO2 fluxes, to facilitate comparison between datasets. This is complicated by the fact that, to first order, different frequencies in the seismic signals associated with gas-driven explosions reflect different processes, with low frequencies being coupled to volume changes and higher frequencies to overpressure. One solution may be to provide integrated absolute
Reference Dalton et al. (2010) Kazahaya et al. (2011) Nadeau et al. (2011) Tamburello et al. (2012) Tamburello et al. (2013) Lopez et al. (2013) Waite et al. (2013) Barnie et al. (2015) Burton et al. (2015) Peters et al. (2015)
amplitudes for all frequencies, low frequencies and high frequencies, respectively, for each seismic event. 3.2. Explosive versus passive degassing The combined capacity to measure gas emissions released over short timescales and from different sources allows the SO2 camera to distinguish and quantify the gas fluxes arising from different styles of volcanic activity. This opens the possibility of new insights into magma dynamics as the relative proportion of gas released during quiescent degassing compared with that in explosions is a direct reflection of the degree of coupling between gas and melt. A summary of the papers where the SO2 camera has been used to distinguish passive degassing fluxes from explosion gas fluxes is shown in Table 2. The first work with the SO2 camera to quantify the relative fluxes of SO2 released via explosive and quiescent degassing was performed by Mori and Burton (2009), who found that for Stromboli 3–8% of the total gas emission was released as explosions. There is an immediate conclusion from this observation: whilst explosions make a lot of noise and receive much attention, they are in fact a second order process compared with the quiescent supply of gas, and magma, to the near surface at Stromboli. This work was followed by that of Kazahaya et al. (2011) on Asama volcano Japan, who also found that the total gas flux was dominated by quiescent degassing, with 84% of SO2 emissions in between explosions. Holland et al. (2011) measured SO2 emissions from Santiaguito, and whilst they did not perform the calculation of the relative contribution of explosive and passive fluxes in that paper, the results they report allow such a calculation to be made. They measured background fluxes of 50 to 85 Mg/day (1 Mg is 1 metric tonne), and an average of ~25 explosions per day each releasing an average of ~650 kg, to produce a daily explosive flux of ~ 16 Mg/day or 19–32% of the total flux. Tamburello et al. (2012) furthered the investigation of degassing styles at Stromboli, quantifying both the emissions from quiescent degassing, ‘puffing’ and explosions. Gas ‘puffing’ behaviour is normally observed from the central of the three craters in the crater terrace of Stromboli, and is not normally associated with explosive activity, so it is a slightly pressurised and modulated quiescent degassing. They report relative proportions of 77% quiescent, 7% explosion and 16% ‘puffing’ SO2 fluxes, thus confirming the domination by quiescent degassing which makes up 93% of the total flux, as reported by Mori and Burton (2009). Waite et al. (2013) report data which allow the relative calculation of quiescent to explosive degassing, and find that on this volcano, anomalously compared with other studies, degassing is dominated by gas released during explosions. From inspection of Figure 4 in Waite et al. (2013) the average total flux over 70 min was of ~1.25 kg/s, producing a total of 5250 kg SO2, whilst 6 explosions in the same period produced 5028 kg, indicating that ~95% of the degassing comes from explosions. Pering et al. (2015) report on SO2 fluxes from mild explosive activity from a vent in the Central Crater of Etna, Italy, whose values are orders of magnitude smaller than the quiescent degassing flux from the main craters of this volcano.
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Table 2 Reports of comparisons between explosive and quiescent degassing fluxes with the SO2 camera. Volcano
Quiescent gas flux %
Explosive gas flux %
Reference
Stromboli, Italy Asama, Japan Santiaguito, Guatemala Stromboli, Italy Fuego, Guatemala Etna, Italy
92–97% 84% 68–81% 93% ~5% N99%
3–8% 16% 19–32% 7% ~95% b1%
Mori and Burton (2009) Kazahaya et al. (2011) Holland et al. (2011) Tamburello et al. (2012) Waite et al. (2013) Peters et al. (2015)
3.3. Comparison of inferred mass of degassed magma with mass of erupted magma One of the earliest uses of SO2 flux measurements, after simple pattern recognition to indicate magmatic unrest, was the quantification of the mass of magma required to produce an observed gas flux. This inferred degassed magma mass can then be compared with the mass of erupted magma. A smaller mass of degassed magma than erupted magma may imply incomplete degassing of magma, but this is very rarely observed. Much more common is the opposite case, where more magma is degassed than is erupted, implying permanent storage of degassed magma under the volcano, leading to endogenous growth. The first work to utilise these mass balance considerations was that of Francis et al. (1993) who examined endogenous growth for Etna, Italy and Kilauea, Hawaii. A key requirement for such calculations is knowledge of the original concentration of sulphur in the melt prior to degassing. This information can best be obtained by analysis of melt inclusions trapped in crystals that grew at high pressure before being rapidly transported to the surface in an eruption. We highlight therefore that the scientific insights which can be gained from SO2 flux measurements can be further enhanced when integrated with other measurements, in this case of the volatile contents of melt inclusions in erupted crystals. Whilst such data exist for some well-studied volcanoes, there are many degassing volcanoes that have not yet been examined for primitive volatile amounts, and therefore such studies should be a priority in order to fully utilise information obtained from the SO2 camera.
3.4. Lava lake dynamics There are four lava lakes recognised on Earth, Erta'Ale in Ethiopa, Nyiragongo in Democratic Republic of Congo, Halemaumau on Kilauea, Hawaii, and Erebus in Antarctica. Lava lakes provide fascinating insights into the dynamics of persistently active volcanoes, revealing styles of degassing, magma supply and magma level which are normally hidden from view. In this issue two papers report the first SO2 camera measurements of SO2 flux from a lava lake, focussing on the Halemaumau crater of Kilauea. Nadeau et al. (2015) examine two examples of lava lake rise and fall. When the lava levels were high, diminished SO2 emissions and volcanic tremor were observed; in contrast, when lava levels were low, degassing rates were higher. The high lava levels were interpreted as resulting from the accumulation of gas below a low-permeability lava lake surface. The recent installation of a permanent SO2 camera to monitor the lava lake at Kilaeua (Kern et al., 2015a, b, see Section 4) will provide an unprecedented insight into the dynamics of this quite rare phenomenon.
Stremme et al. (2012) used the scanning infrared emission spectrometer based SIGIS system from Bruker optics (www.bruker.de) to measure the SiF4/SO2 ratio in the volcanic plume of Popocatepetl in Mexico. They found distinct trends related to volcanic activity, with SiF4 increasing relative to SO2 prior to an explosion on the volcano, and then decreasing thereafter. The passive, thermal emission spectroscopy upon which the SIGIS system is based can be used during the day and the night, a major advantage over UV-based systems which require scattered sunlight. Platt et al. (2015) give a full analysis of the various technologies that can be used in SO2 camera systems. BrO/SO2 and OClO/SO2 ratios are reported by General et al. (2015), who used the IDOAS approach to detect these difficult-to-measure volcanic gases. Pering et al. (2014) simultaneously measured gas compositions and SO2 fluxes with MultiGas and SO2 camera instruments respectively on Mt. Etna (Italy). Lopez et al. (2013) report ash quantification in an explosion from Karymsky (Kamchatka), using the infrared imaging system NicAIR. This work was extended by Lopez et al. (2015), examining ash and SO2 quantifications on both Stromboli (Italy) and Karymsky (Russia). Cerminara et al. (2015) examine the integration of thermal imagery with a numerical model to determine ash loading in explosions at Stromboli. 4. Implementation of SO2 cameras in monitoring networks As described in Section 3, whilst flux determinations may appear simple in theory, in practice there are many challenges in producing the accurate and precise SO2 slant column amounts and fluxes required to perform useful comparisons with geophysical data (Section 3.1) and erupted magma masses (Section 3.3). However, volcano observatories require instruments that can be relied upon to produce good-quality data, without the need for experts in radiative transfer. There is therefore a tension between what is actually required by volcano observatories, and what is currently provided by SO2 flux monitoring systems that currently do not resolve a host of issues related to plume geometry, plume speed and multiple scattering. Furthermore, in an ideal world, volcano observatories would have 24-hour SO2 flux monitoring capabilities, which cannot be provided by the UV-based SO2 cameras (or scanning systems, see Galle et al., 2010), which make up the majority of instruments to date. An investment in infrared-based technique development (or other techniques, see Platt et al., 2015) may well yield great benefits for volcano observatories, greatly extending their monitoring capacities. The first fully automated SO2 camera systems have been deployed on two volcanoes to date, Kilauea (Kern et al., 2015a, b) and Stromboli (Burton et al., 2015). These imaging systems represent a stepping stone to the future of SO2 flux monitoring at volcano observatories, and are already producing unprecedentedly detailed insights into the degassing processes at both volcanoes.
3.5. Gas composition and ash detection/quantification 5. Conclusions As well as revealing the SO2 flux from volcanoes, infrared-based SO2 cameras can also be used to determine plume gas compositions and ash burdens, whilst scanning 2D UV spectrometers can be used to measure several trace volcanic gases (like BrO and OClO).
The development of the SO2 camera is a major step forward for volcano research, enabling comparison of quantified gas emissions with geophysical data (see Section 3.1) and erupted masses (Section 3.3).
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Such comparisons require precise and accurate measurements of SO2 flux, which are far from trivial to achieve, due to the challenges of light scattering and complex flow within the advecting plume. Solutions to these problems are beginning to be within our reach however (see Section 2). Multi-spectral imaging can open the door to even more detailed studies including the quantification of gases other than SO2, allowing examination of plume gas chemistry (see Section 3.5). Some of the major benefits of SO2 flux monitoring with SO2 cameras arise from comparisons between the flux of SO2 released explosively versus that released passively, as this reflects on the coupling between gas and magma, a major controlling factor in the overall behaviour of a volcano (Section 3.2). The SO2 camera has revolutionised our ability to monitor volcanic gas fluxes, and a thriving community has arisen around this technology. The May 2013 ESF-funded MeMoVolc workshop has allowed this community to come together in the same place for the first time, and resulted in this special issue, which reflects the excitement and potential provided by this new technique. Acknowledgements We gratefully acknowledge the European Science Foundation which provided the funding to the MeMoVolc project, kindly managed by Prof. Tim Druitt and Dr. Augusto Neri, whom we thank for their support in creating the Plume Imaging Workshop, grant number 4799. We also thank all the participants of the Plume Imaging Workshop for making it such a stimulating and productive event, and for their continued enthusiasm since then, which has resulted in this Special Issue. Prof. Lionel Wilson and Prof. Clive Oppenheimer are thanked for their respective editing and revision of this work. We thank Patrizia Pantani, Raffaella Pignolo and Gemma Prata for their assistance in organising the workshop. The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement no. [279802]. References Barnie, T., Bombrun, M., Burton, M.R., Harris, A.J.L., Sawyer, G., 2015. Quantification of gas and solid emissions during Strombolian explosions using simultaneous sulphur dioxide and infrared camera observations. J. Volcanol. Geotherm. Res. 300, 167–174. Burton, M.R., Sawyer, G., 2013. iFit and light dilution: ultraviolet volcanic SO2 measurements under the microscope. EGU General Assembly Conference Abstracts, p. 10715. Burton, M., Allard, P., Mure, F., La Spina, A., 2007. Magmatic gas composition reveals the source depth of slug-driven Strombolian explosive activity. Science 317 (5835), 227–230. Burton, M.R., Caltabiano, T., Mure, F., Salerno, G., Randazzo, D., 2009. SO2 flux from Stromboli during the 2007 eruption: results from the FLAME network and traverse measurements. J. Volcanol. Geotherm. Res. 182 (3–4), 214–220. Burton, M.R., Salerno, G.G., D'Auria, L., 2015. SO2 flux monitoring at Stromboli with the new permanent INGV SO2 camera system: A comparison with the FLAME network and seismological data. J. Volcanol. Geotherm. Res. 300, 95–102. Campion, R.A., Delgado-Granados, H., Mori, T., 2015. Image-based Correction of the Light Dilution Effect for SO2 Camera Measurements. J. Volcanol. Geotherm. Res. 300, 48–57. Cerminara, M., Ongaro, T.E., Valade, S.A., Harris, A.J.L., 2015. Volcanic plume vent conditions retrieved from infrared images: A forward and inverse modeling approach. J. Volcanol. Geotherm. Res. 300, 129–147. Chouet, B., Dawson, P., Ohminato, T., Martini, M., Saccorotti, G., Giudicepietro, F., De Luca, G., Milana, G., Scarpa, R., 2003. Source mechanism of explosions at Stromboli Volcano, Italy, determined from moment-tensor inversions of very-long-period data. J. Geophys. Res. 108 (B1), 2019. http://dx.doi.org/10.1029/20042JB001919. Dalton, M.P., Waite, G.P., Watson, I.M., Nadeau, P.A., 2010. Multiparameter quantification of gas release during weak Strombolian eruptions at Pacaya Volcano, Guatemala. Geophys. Res. Lett. 37. Edmonds, M., Herd, R.A., Galle, B., Oppenheimer, C.M., 2003. Automated, high time resolution measurements of SO2 flux at Soufriere Hills Volcano, Montserrat. Bull. Volcanol. 65, 578–586.
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